Re: [PATCH] mm: migrate: fix an incorrect call of prep_transhuge_page()

["Zi Yan" <zi.yan@cs.rutgers.edu>]

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On 22 Nov 2017, at 5:14, Michal Hocko wrote:

> On Wed 22-11-17 10:35:10, Michal Hocko wrote:
> [...]
>> Moreover I am not really sure this is really working properly. Just look
>> at the split_huge_page. It moves all the tail pages to the LRU list
>> while migrate_pages has a list of pages to migrate. So we will migrate
>> the head page and all the rest will get back to the LRU list. What
>> guarantees that they will get migrated as well.
>
> OK, so this is as I've expected. It doesn't work! Some pfn walker based
> migration will just skip tail pages see madvise_inject_error.
> __alloc_contig_migrate_range will simply fail on THP page see
> isolate_migratepages_block so we even do not try to migrate it.
> do_move_page_to_node_array will simply migrate head and do not care
> about tail pages. do_mbind splits the page and then fall back to pte
> walk when thp migration is not supported but it doesn't handle tail
> pages if the THP migration path is not able to allocate a fresh THP
> AFAICS. Memory hotplug should be safe because it doesn't skip the whole
> THP when doing pfn walk.
>
> Unless I am missing something here this looks like a huge mess to me.

+Kirill

First, I agree with you that splitting a THP and only migrating its head page
is a mess. But what you describe is also the behavior of migrate_page()
_before_ THP migration support is added. I thought that was intended.

Look at http://elixir.free-electrons.com/linux/v4.13.15/source/mm/migrate.c#L1091,
unmap_and_move() splits THPs and only migrates the head page in v4.13 before THP
migration is added. I think the behavior was introduced since v4.5 (I just skimmed
v4.0 to v4.13 code and did not have time to use git blame), before that THPs are
not migrated but shown as successfully migrated (at least from v4.4’s code).

Naoya and I had a discussion on this ‘splitting a THP and migrating its head page’ before.
We think we should try to spilt the THP and migrate all its subpages. I did not have
time to get the code out yet.

I am traveling today, so I may not be able to do anything useful. I will be on a break
for a month and will not have good accesses to any machines. I can try to fix
this ‘splitting a THP and migrating its head page’ after that.

—
Best Regards,
Yan Zi

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